1. Field of the invention
The invention relates to a chemical process for denitrification of water and, more particularly, to a process for selective denitrification of water in the presence of sulfate ions.
2. Background of the prior art
The importance of a water treatment process for the removal of nitrate ion is evident from the many problems with which it is associated. From a health standpoint, the medical literature documents excess levels of nitrate ions as a proximate cause of methemoglobinemia, a disease of the newborn, and suggests a potential link between nitrate ion and stomach cancer in humans. Aside from health considerations, nitrate pollution is responsible for poisoning lakes and rivers with resulting harm to fish and other aquatic life and causing algae "blooms", a direct result of the nutrient function of nitrogen. Profusion of the algae "blooms" may serve as an obstacle impeding the flow of water traffic and may pose a significant problem in reservoir storage.
Prior art techniques and processes for denitrification of water fall into three basic categories: biological, desalting, and chemical.
There are numerous biological treatments for effecting denitrification of water. The disadvantages relative to chemical methods are slow reaction times, large holding tanks, and decreased biological oxygen demand (BOD), a measure of the biodegradable organic content of the water. Also, biological organisms essential for such processes cannot survive high TDS waters such as certain brine streams.
Conventional desalting techniques such as reverse osmosis, ion-exchange, and electrodialysis, although efficient in many respects, invariably are too expensive in many applications, particularly where the volume of natural or reclaimed water to be treated is large and a cheap source of electricity is not available. For the most part, conventional desalting techniques lack specificity for nitrate relative to chloride or sulfate. This can be an important cost consideration.
Chemical processes for denitrification of water have involved treatment with various reactive metals and compounds. These reactions can be understood to produce nascent hydrogen which can then act as a reducing agent for anions such as nitrate.
A process which falls into this category is that disclosed in U.S. Pat. No. 3,617,579. In this process a partial denitrification is achieved by treating a dilute nitrate ion aqueous solution with ferrous ion in the presence of a catalytic quantity of cupric ion at an initial pH between 7 and 11. The patent teaches that the pH in general drops during the course of the reaction and should be adjusted preferably to a value between 7 and 9 by, for example, the use of lime or sodium hydroxide. The preferred source of ferrous ion is stated as ferrous sulfate. The patent also teaches that any source of cupric ion catalyst may be employed and cites cupric sulfate as an example. While the exact pressure and temperature is not regarded as critical, the patent does teach that anaerobic conditions are preferred to avoid oxidation of the ferrous ion by air. Absent anaerobic conditions, extra ferrous ion is required to compensate for the degree of oxidation that occurs.
Another process in the category of chemical denitrification is that disclosed in U.S. Pat. No. 4,642,192. This patent relates to the treatment of water, particularly drinking water, with a metal particulate matter such as aluminum, iron, steel, zinc, or copper, or mixtures or alloys thereof, to provide removal of undesirable contaminants such as chlorine and nitrates. The metal particulate matter is selected on the basis of its redox potential relative to the redox potential of the undesirable constituent such that it favors a spontaneous oxidation-reduction reaction between the metal and the undesirable constituent. The patent teaches regulating the pH of the fluid prior to treatment to enhance the removal of contaminants having pH dependent oxidation-reduction activities. Specific examples disclosed in the patent show that the patented process effected a decrease in the level of dissolved nitrates in water when the fluid medium was at least slightly acidic such as having a pH of 6.5 or less.
The prior art chemical processes for denitrifying water are generally attended by a number of disadvantages. For example, the use of ferrous hydroxide as a reductant, as taught by U.S. Pat. No. 3,617,579, shows appreciable decrease in reaction rate due to dissolved oxygen. In order to compensate for this decrease, the patentee advocates conducting the reaction at anaerobic conditions, thus necessitating high capital equipment costs since the reaction must be conducted in a closed container under an inert gas, such as nitrogen or helium. Absent anaerobic conditions, the patentee must employ added reductant, thereby adding to the chemical costs of the process.
In many prior art processes the treatment produces a precipitate or sludge as a by product This presents a disposal problem with its attendant costs. Where the disposal relates to hazardous waste the problem is compounded.
A further disadvantage of many prior art processes is the need for additional water treatment steps. In this regard, the introduction of ferrous ion in the process of U.S. Pat. No. 3,617,579 may very well exceed the maximum contaminant level for iron and copper in drinking water as specified by the U.S. Environmental Protection Agency, thereby necessitating the added expense of removal of these two elements.